When selecting a trolling motor, there are two important considerations: power (thrust and voltage) and shaft size. Having enough power will ensure you can maneuver and propel your fishing boat at a sufficient speed. Having the correct shaft size will ensure your trolling motor does not cavitate and scare away fish.

Determining Your Power Requirements

As a rule of thumb, a fishing boat requires 2 pounds of thrust for ever 100 pounds of weight (including equipment, gear, and passengers). Weather conditions like wind and current also factor into power requirements. If you’re going to be working against the wind or current, you’ll need a trolling motor with a little bit more thrust.

When determining power requirements you’ll also want to take into account battery capacity. Larger motors typically require more battery capacity. If you plan on fishing the entire day with a 16′ boat or larger, you’ll likely need a 24V battery. If you’re really serious, go with a 36V motor.

Selecting the Correct Shaft Length

Shaft length is another important consideration when selecting and setting up your trolling motor. An incorrect shaft length will cause an electric trolling motor to cavitate loudly and scare away fish. When determining correct shaft length, the general rule is that foot (lower unit) of the motor should be cover by a minimum of 12″ of water.

Having the correct shaft length is more critical for bow-mount trolling motors than it is for transom-mount motors. For most fishing boats, the transom is going to be about the same distance to the water. Consequently, a standard transom shaft length will work for most fishing boats.

However, the distance from the bow to the water varies greatly depending on boat make and style. Consequently, shaft length requirements for bow-mounted motors vary greatly.

Measuring for your Correct Shaft Length

A general approach to determining shaft length is to measure from the mounting surface of the bow or transom where the motor sits to the surface of the water. Now take that measurement and add 16″ (for MotorGuide) or 20″ (for Minn Kota) for a recommended minimum shaft length. (For other models, check your buyer’s guide. If you will regularly fish in rough water, add 5″ to the measurement. Add 12″ to the measurement if you will be using a hand-controlled trolling motor while standing.

You can also use the following measurement tables to determine recommended shaft length based on bow to waterline and transom to waterline measurements.

A standard shaft length for most fishing boats is 42″.